Power meters require the user to select a carrier frequency before making power measurements. This frequency setting is mapped to a correction factor that is be applied to all subsequent power measurements. A typical graph 100 showing correction or calibration factor 101 versus frequency 103 is shown in FIG. 1. The graph 100 is for a 8481A thermocouple sensor from Agilent Technologies, Inc. of Santa Clara, Calif., USA. Modern diode power sensors, such as those used to characterize pulsed RF systems, require even more complex measurement corrections. These can vary considerably over frequency and make the selection of the proper carrier frequency even more important.
Frequency hopping signals have existed for many years. Frequency hopping moves, or hops, the carrier frequency of a signal between an upper and lower frequency range a number of times per second. This tends to spread out the signal's spectrum over that frequency range. Frequency hopping provides resistance to jamming and low probability of intercept which is useful in military radar and communication applications.
One current communication standard that makes use of frequency hopping is Bluetooth. Bluetooth has 79 channels each 1 MHz wide and hops from one channel to another in a pseudorandom manner 1600 times per second. Frequency hopping is also used in cordless phones following the WDCT standard and the HomeRF standard which provides for a broader range of interoperable consumer devices.
When using a power meter to measure frequency hopping signals, due to the nature of the signal which hops its carrier frequency within a range of frequencies, the measurement will have error if the frequency of the power meter is set to a single value. Average responding thermal sensors will not show too much degradation in accuracy. They will, however only report the average power of the transmission as it hops through all frequencies.
A bigger problem is encountered when the power at each frequency in the hopping sequence has to be checked. Even with fast pulse trains, some power meters have the capability to capture and analyze every pulse in a pulse train. Due to the complexity of the applied corrections there has been only one set of frequency corrections applied and thus, if all pulses are captured, there is an uncertainty added due to the wrong frequency correction being applied. One solution has been to set the power meter to the desired frequency and delay the acquisition until the point in the hopping sequence when that frequency is transmitted. This is repeated until the measurements at all desired frequencies are completed. However, this solution is very slow as it requires that the sequence be repeated N times in order to measure the power at all N frequencies in the sequence.
In addition to signal types that are designed to be hopping there is also the common component test scenario where the test signal is stepped through a sequence of frequencies. This type of testing is often used to measure the gain of amplifiers, for example, as a function of frequency. These test scenarios suffer from problems similar to those of frequency hopping signals as in both scenarios obtaining the results is slow or the accuracy is compromised.